System and method for estimating a pointing error of a satellite antenna

ABSTRACT

A system for estimating a pointing error of an antenna (ANT) of a satellite, the satellite includes a payload (CU) comprising a multichannel transmitter or receiver comprising a multichannel antenna (ANT), one analogue processing chain per channel and a set of digital integrated circuits (PN), the system comprising an estimation device (EST), implemented aboard the satellite or in a ground station, for estimating a pointing error of the antenna, the device for estimating a pointing error being configured to: acquire, for at least two channels of the transmitter or of the receiver, at least two test signals, each test signal having been transmitted or received by the antenna along a different direction (θ A , θ B ), for at least one pair of acquired test signals, determine, for each test signal, a relative complex gain between the test signal received or transmitted respectively on two distinct channels, determine a comparative measurement between the two test signals from either the ratio between the two relative complex gains and/or the difference between the phases of the two relative complex gains, determine a pointing error (dθ) of the antenna on the basis of the comparative measurement, of the expected directions of transmission or of reception of the test signals (θ A , θ B ) and of a model of the gain of the antenna for each channel and in a plurality of directions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to foreign French patent applicationNo. FR 1903183, filed on Mar. 28, 2019, the disclosure of which isincorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to the field of satellite communication systemsand more precisely that of active antennas installed on board asatellite and operating in transmission mode or in reception mode.

The invention pertains to a system and a method for estimating apointing error of a satellite antenna.

BACKGROUND

The invention proposes a solution to the problem of the precisedetermination of the pointing error of an active antenna aboard asatellite, in particular, but not solely, in the case of antennas withreflector.

The pointing error of an antenna corresponds to the difference betweenthe direction of sighting of the antenna which is controlledmechanically and the actual direction. In practice, a disparity mayexist between the desired direction and the actual direction notably onaccount of the thermo-elastic deformation of the mechanical links of theantenna, of the dynamic deformation of the satellite and of its angularstability or of imperfect attitude.

Indeed, the direction of pointing of the antenna is determined in areference frame tied to the satellite, on the basis of the attitude ofthe satellite. However, the determination and maintenance of theattitude of the satellite is possible only with a non-zero error margin,typically of the order of 0.1° over a day. This error carries over tothe pointing precision of the antenna, thereby impacting the directionof arrival of the signals received or transmitted by the antenna of thesatellite, relatively and respectively to a reference frame of theantenna or to a reference frame on the ground.

This lack of precision may be harmful for certain applications. Notably,when one wishes to carry out a precise calibration of the variousprocessing channels of a payload, the direction of pointing of theantenna must be known precisely.

Moreover, in the case of multibeam antennas which illuminate spots ofrestricted size, a need also exists to finely control the pointing ofthe antenna so as to ensure the stability of the antenna coverage.

Various solutions exist for precisely estimating the pointing error of asatellite antenna.

A first solution consists in equipping the antenna with a so-called “RFsensing” system dedicated to the measurement of pointing consisting of asmall number of sources, or indeed with a specific antenna, and withelectronic processing equipment. This system must be mechanicallycoupled to the antenna.

A drawback of this solution is that it involves an increase in the massand the consumption of the satellite. It also makes it necessary toensure that the additional sources do not disturb the behaviour of theantenna. The consequences in terms of bulkiness, mass and consumptionmay turn out to be non-negligible.

A second solution consists in neglecting the thermo-elastic effects onthe mechanical links of the antenna. The pointing of the antenna is thenestimated on the basis of the satellite's attitude, which is estimatedwith optical sensors (of the “star tracker” type) carrying outmeasurements on stars. This solution although economical does not makeit possible to obtain the expected precision in the pointing of theantenna.

A third solution consists, in the case of an antenna in reception mode,in transmitting several signals from various ground stations at variousgeographical positions, on known frequencies, and then in carrying out,in the digital processor of the payload, the coherent acquisition of thesignals arising from the various radiofrequency chains downstream of theantenna. On the basis of the sequences digitized over the set ofchannels, it is possible to implement an algorithm of MUSIC type, basedon an estimation of the correlation matrix of the signals and anoise/signal eigen sub-space decomposition. This makes it possible toestimate the directions of arrival of the signals transmitted by theground stations.

However, this technique is known to be very sensitive to an unfavourablesignal-to-noise ratio and above all to the imperfections in terms ofdelay, phase and gain of the radiofrequency chains.

Moreover, this procedure is difficult to apply for an antenna intransmission mode, since it is necessary to distinguish the signalstransmitted by the diverse radiating elements of the antenna,transmitted simultaneously, within the signal received on the ground, soas to construct a correlation matrix. The separation of the signalsdegrades the precision of the measurements in the case of a large numberof radiating elements transmitting simultaneously.

SUMMARY OF THE INVENTION

The invention proposes a novel solution to the problem of estimating thepointing error of an antenna in reception mode or in transmission mode.

It is based on employing at least two ground stations transmitting orreceiving a test signal along two different directions. The signal isprocessed on board or on the ground so as to precisely determine thedirection of pointing of the antenna on the basis of a model of the gainof the antenna in various directions.

The proposed solution makes it possible to improve the precision of theestimation of the pointing of the antenna, without requiring anyspecific means on board the satellite impacting mass and consumption.The precision obtained is independent of the dispersions betweenanalogue radiofrequency chains of the payload. It is also independent ofthe variations of the propagation conditions between the signalstransmitted by the various ground stations and of the dispersions of theanalogue radiofrequency chains of the ground stations. Also, thesolution has no impact on the service rendered by the satellite. Itallows regular or indeed continuous estimation of the antennamispointing. The level of precision of the estimation can be furtherimproved by utilizing more ground stations or various frequencies orelse several reception/transmission channels aboard the satellite.

The subject of the invention is a system for estimating a pointing errorof an antenna of a satellite, the satellite comprising a payloadcomprising a multichannel transmitter or receiver comprising amultichannel antenna, one analogue processing chain per channel and aset of digital integrated circuits, the system comprising an estimationdevice, implemented aboard the satellite or in a ground station, forestimating a pointing error of the antenna, the device for estimating apointing error being configured to:

-   -   acquire, for at least two channels of the transmitter or of the        receiver, at least two test signals, each test signal having        been transmitted or received by the antenna along a different        direction from the viewpoint of the satellite,    -   for at least one pair of acquired test signals, determine, for        each test signal, a relative complex gain between the test        signal received or transmitted respectively on two distinct        channels,    -   determine a comparative measurement between the two test signals        from either the ratio between the two relative complex gains        and/or the difference between the phases of the two relative        complex gains,    -   determine a pointing error of the antenna on the basis of the        comparative measurement, of the expected directions of        transmission or of reception of the test signals and of a model        of the gain of the antenna for each channel and in a plurality        of directions.

According to a particular aspect of the invention, each test signal iscomposed of at least one spectral line.

According to a particular aspect of the invention, the device forestimating a pointing error is configured to determine a plurality ofestimates of a pointing error on the basis of several pairs of testsignals transmitted or received in different directions or severaldifferent pairs of distinct channels or several frequencies of testsignals.

According to a particular aspect of the invention, the device forestimating a pointing error is configured to determine a pointing errorof the antenna by:

-   -   determining, on the basis of the model of the gain of the        antenna, a model of comparative measurement between the two test        signals, dependent on a pointing error of the antenna, on the        frequency and on the directions of transmission or of reception        of the test signals (θ_(A), θ_(B)),    -   searching for the value of the pointing error which makes it        possible to minimize the difference between the comparative        measurement and the comparative measurement model taken at this        value.

According to a particular aspect of the invention, the payload comprisesa multichannel receiver and the device for estimating a pointing erroris configured to receive, for at least two reception channels, adigitized temporal sample sequence of at least two test signals, thetemporal sample sequence being tapped off simultaneously from thevarious channels in the set of digital integrated circuits.

According to a particular aspect of the invention, the system forestimating a pointing error furthermore comprises at least two testground stations, each test ground station being configured to transmit atest signal towards the satellite.

According to a particular aspect of the invention, each test groundstation is configured to transmit the test signals in turn on one andthe same frequency or simultaneously on frequencies which aresufficiently close to be affected by the same errors.

According to a particular aspect of the invention, the payload comprisesa multichannel transmitter, the device for estimating a pointing errorbeing implemented in a ground station, each test signal received by adifferent ground station being transmitted by the antenna along adifferent direction of transmission from the viewpoint of the satellite.

According to a particular aspect of the invention, the system forestimating a pointing error furthermore comprises at least two testground stations, each test ground station being configured to:

-   -   receive a test signal transmitted by the satellite along a        different direction from the viewpoint of the satellite,    -   separate the test signal received on the ground into several        signals corresponding to the transmission channels of the        satellite,    -   dispatch, to the device for estimating a pointing error, the        test signals received for at least two distinct transmission        channels.

According to a particular aspect of the invention, the system isconfigured to apply a procedure for multiple access to the test signalstransmitted by the satellite.

The subject of the invention is also a method for estimating a pointingerror of an antenna of a satellite, the satellite comprising a payloadcomprising a multichannel transmitter or receiver comprising amultichannel antenna, one analogue processing chain per channel and aset of digital integrated circuits, the method comprising the steps of:

-   -   acquiring, for at least two channels of the transmitter or of        the receiver, at least two test signals, each test signal having        been transmitted or received by the antenna along a different        direction,    -   for at least one pair of acquired test signals, determining, for        each test signal, a relative complex gain between the test        signal received or transmitted respectively on two distinct        channels,    -   determining a comparative measurement between the two test        signals from among the ratio between the two relative complex        gains and/or the difference between the phases of the two        relative complex gains,    -   determining a pointing error of the antenna on the basis of the        comparative measurement, of the expected directions of        transmission or of reception of the test signals and of a model        of the gain of the antenna for each channel and in a plurality        of directions.

According to a variant, the method according to the inventionfurthermore comprises a step of correcting the pointing of the antennaon the basis of the pointing error.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings illustrate the invention:

FIG. 1 represents a diagram of a system for estimating a pointing errorof an antenna of a satellite operating in reception mode according to afirst embodiment of the invention,

FIG. 2 represents a flowchart detailing the steps of carrying out amethod for estimating a pointing error with the aid of the system ofFIG. 1,

FIG. 3 represents a diagram of a system for estimating a pointing errorof an antenna of a satellite operating in transmission mode according toa second embodiment of the invention,

FIG. 4 represents a flowchart detailing the steps of carrying out amethod for estimating a pointing error with the aid of the system ofFIG. 3.

DETAILED DESCRIPTION

FIGS. 1 and 2 illustrate the implementation of a system and of a methodfor estimating the pointing error of an antenna of a satellite, inreception mode.

The system described in FIG. 1 comprises a payload CU of a satellite inorbit and at least two ground stations ST₁, ST₂.

Each ground station ST₁, ST₂ comprises notably an antenna pointedtowards the satellite, a transmission chain for communicating with thesatellite and a digital-to-analogue converter. In the case where themethod according to the invention is implemented on the ground, at leastone ground station ST₂ or some other remote equipment interfaced withthe ground station ST₂ comprises a reception chain, ananalogue-to-digital converter, a means of communication with thesatellite via a secure link LS, a memory for storing the digitizedsignal received and a computation device for processing the signal. Thelink LS is made secure, for example by employing an error protectionmechanism of the error-correcting code type.

The payload CU comprises one or more antenna(s) ANT or antenna arrayconsisting of several radiating elements. Each radiating elementreceives a signal which is processed in the payload CU by a processingchannel. The payload CU thus exhibits multichannel operation. A channelcorresponds to a radiating element of the antenna ANT. In FIG. 1, forthe sake of simplicity, a payload comprising two reception channels hasbeen represented, but the number of channels is in general bigger.

Each processing channel comprises an analogue radiofrequency chain RF₁,RF₂ which consists of one or more filter(s), of one or more amplifier(s)and optionally of one or more mixer(s) for carrying out a frequencytransposition of the signal received.

At the output of each analogue radiofrequency reception chain, ananalogue-to-digital converter ADC₁, ADC₂ is positioned so as to convertthe analogue signals into digital signals which are provided to a set ofdigital integrated circuits PN. Each digital integrated circuit is, forexample, an integrated circuit specific to an application (also known bythe English acronym ASIC for “Application-Specific Integrated Circuit”)or an array of in situ programmable gates (also known by the Englishacronym FPGA for “Field-Programmable Gate Array”) or a set of logicgates or else a signal processor or a generic processor.

The set of digital integrated circuits PN comprises, for example, onedigital filter per processing channel and a beamforming circuit FFCcarrying out a beamforming function common to a set of the processingchannels. The function of the beamforming circuit FFC is notably tolinearly combine the signals received with a particular set of complexgains (amplitude and phase) to form a beam in the desired direction, inrelation to a reference frame of the antenna, so as to receive theuseful signals. The particular combination of the signals received withcomplex gains constitutes a law of combination or a beam-forming law.

The example of FIG. 1 is given purely by way of nonlimitingillustration. In particular, the set of digital integrated circuits canexhibit a modular architecture different from that of FIG. 1. Forexample, it can comprise several series of digital integrated circuitsfor each channel or several beamforming integrated circuits operating inparallel and in cascade for sub-groups of channels.

In all architecture cases, the set of digital integrated circuitscomprises a means MEM, for example a memory, for capturing or recording,in a coherent manner over one and the same time interval, the digitalsignals of a set of the channels, for example at the output of theanalogue-to-digital converters ADC₁, ADC₂. In one embodiment of theinvention, the set of digital integrated circuits PN furthermorecomprises a device EST for estimating the pointing error of the antennaANT on the basis of the digital signals captured by the memory MEM. Thisdevice EST is embodied, for example, by means of a computer that isinstalled on board and includes software elements and/or hardwareelements. In another embodiment of the invention, the device EST forestimating the pointing error is located in a ground station ST₁, ST₂ orin some other equipment on the ground.

The method according to the invention starts with the dispatching 201 ofat least two test signals by at least two ground stations, in twodifferent directions of arrival θ_(A), θ_(B). Without departing from thescope of the invention, more than two ground stations can be used, eachtransmitting a test signal towards the satellite from a differentdirection. The directions of arrival must be different from theviewpoint of the satellite. To satisfy this constraint, the groundstations are, for example, situated in different spots. A spotcorresponds to a beam of the multibeam antenna coverage.

The test signals are narrowband signals which consist, for example, of aspectral line at a given frequency making it possible to not interferewith the useful signals. The test signals can also consist of carriersmodulated by another predetermined signal. The ground stations ST₁, ST₂transmit in turn on one and the same frequency or simultaneously onfrequencies which are different but which are sufficiently close to beaffected in the same manner by the propagation, the RF chains and theantenna. The test signals are known beforehand to the satellite and tothe ground stations.

The signals received by the satellite are processed on board by theradiofrequency chains RF₁, RF₂ of each channel and are then digitized bythe converters ADC₁, ADC₂.

In a second step 202 of the method according to the invention, thedigitized signals are acquired in a coherent manner over one and thesame time interval for at least two distinct channels. For example, thesignals are tapped off directly at the output of the analogue-to-digitalconverters of two channels or somewhere else in the set of digitalintegrated circuits, the only constraint being that the signalscorresponding to two distinct channels be able to be tapped offsimultaneously.

On the basis of the signals digitized simultaneously on at least twodistinct channels and for at least two test signals received in twodifferent directions of arrival, the following steps of the methodaccording to the invention are aimed at determining an estimate of thepointing error of the antenna. These steps can be carried out on boardthe satellite in the set of digital integrated circuits PN or by acomputer which is installed on board or on the ground in one of theground stations ST₁, ST₂ used for the transmission of a test signal orin some other equipment. FIG. 1 shows diagrammatically the case wherethe computation of the pointing error is carried out by an estimationdevice EST which is implemented in the set of digital integratedcircuits PN.

For each of the at least two channels, a step of digital filtering ofthe signal around the useful carrier of the test signal makes itpossible to recover precisely the spectral line transmitted by theground stations.

Thereafter, in a step 203, for each test signal received correspondingto the transmissions by the two ground stations ST₁, ST₂, a relativecomplex gain between two reception channels of the payload isdetermined. The relative complex gain is determined by performing anintercorrelation computation between the digital signal of the firstchannel and the digital signal of the second channel. Indeed, thecalibration signal being propagated identically on each channel, theresult of the intercorrelation of the signals makes it possible toobtain a relative gain which contains the relative gain and the relativephase between the two processing channels. The intercorrelationcomputation is, for example, carried out either directly in the timedomain by correlation computation, or indirectly in the frequency domainby means of two direct Fourier transforms, of a complex conjugation, ofa complex multiplication and of an indirect Fourier transform, accordingto techniques known to the person skilled in the art.

Let dθ be the pointing error of the antenna ANT.

For the ground station ST₁, seen by the antenna ANT in the directionθ_(A)+dθ, a first measurement of relative complex gain, for a frequencyf of the test signal, is thus carried out:

Mes _(A)(ρ_(k,1))=ρ_(k,1)(Antenna_(f,θ) _(A)_(+dθ))*ρ_(k,1)(RF_(f))*ρ_(k,1)(DAC)  [Math. 1]

For the ground station ST₂, seen by the antenna ANT in the directionθ_(B)+dθ, a second measurement of relative complex gain, for a frequencyf of the test signal, is thus carried out:

Mes _(B)(ρ_(k,1))=ρ_(k,1)(Antenna_(f,θ) _(B)_(+dθ))*ρ_(k,1)(RF_(f))*ρ_(k,1)(DAC)  [Math. 2]

Each measurement of relative complex gain corresponds to the product ofseveral terms. The terms ρ_(k,1)(Antenna_(f,θ) _(A) _(+dθ)) andρ_(k,1)(Antenna_(f,θ) _(B) _(+dθ)) correspond to the gain of the antennain the respective directions θ_(A)+dθ and θ_(B)+dθ, for the frequency f.

The terms ρ_(k,1)(RF_(f)) and ρ_(k,1)(DAC) correspond respectively tothe gain ratios between the two channels, related to the differences inbehaviour of the radiofrequency chains for the frequency f and of theanalogue-digital converters of these two channels. These terms arecommon to the two measurements.

In a following step 204, the ratio between the two measurements ofrelative gain that were determined in step 203 is determined.

$\begin{matrix}{{RMes_{B/A}},k,{1 = \frac{\rho_{k,1}\left( {Antenna}_{f,{\theta_{B} + {d\; \theta}}} \right)}{\rho_{k,1}\left( {Antenna}_{f,{\theta_{A} + {d\; \theta}}} \right)}}} & \left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack\end{matrix}$

By computing this ratio, the terms which are common to the twomeasurements offset one another and only the contributions related tothe gain of the antenna are retained.

In another step 205, the ratio computed in step 204 is thereaftercompared with a model of the gain of the antenna dependent on thedirection of arrival of the signal and on the frequency. On the basis ofthis model, of the ratio computed in step 204 and of the values of theexpected directions of arrival of the test signals, an estimate of thepointing error dθ is deduced therefrom.

The directions of arrival θ_(A), θ_(B) are determined on the basis ofthe known positions of the ground stations ST₁, ST₂ and of thesatellite, and of the reference frame of the antenna of the satellite.

A possible procedure for determining the pointing error dθ consists insearching for the value of dθ which makes it possible to minimize theerror between the aforementioned ratio determined in step 204 and theratio of the antenna gains which were determined on the basis of themodel for the respective directions θ_(A)+dθ and θ_(B)+dθ. Any suitablenumerical solution procedure makes it possible to determine an estimateof the value of dθ.

In a last step 206, the estimated pointing error is used to correct thepointing of the antenna either mechanically, or by adapting thebeamforming laws to integrate the estimated mispointing dθ.

In a variant embodiment of the method according to the invention,several estimates of the pointing error dθ are determined.

For example, more than two ground stations can be used, in this case,several ratios of relative gains are computed for various pairs of testsignals received along different directions of arrival.

Another solution consists in utilizing more than two reception channelsin the payload. In this case, several ratios of relative gains arecomputed for various pairs of reception channels.

Finally, several test signals can be transmitted sequentially ondifferent frequencies. In this case, several ratios of relative gainsare computed for various frequencies.

The three variants described hereinabove can be combined together(several ground stations, several reception channels and severalfrequencies).

An advantage in using several estimates of the pointing error is thatthis makes it possible to lift potential ambiguities, for example ifseveral values of the error dθ satisfy equality between the ratio ofgain measurements and the ratio of the gain values which were determinedon the basis of the antenna gain model.

In another variant embodiment of the method according to the invention,only the phase or only the amplitude of the relative complex gains areutilized. In this case, the ratio of complex gains which was computed instep 204 is replaced with a ratio of amplitudes or with a difference ofphases.

FIGS. 3 and 4 describe another variant embodiment of the inventionapplied to an antenna in transmission mode.

FIG. 3 represents a payload CU of a satellite in orbit comprising anantenna ANT operating in transmission mode. The payload CU comprises aset of digital integrated circuits PN carrying out a beamformingfunction in transmission mode on N channels, digital-to-analogueconverters at the output of each channel, RF transmission chainsconnected between the outputs of the digital-to-analogue converters andone or more antenna(s) ANT or antenna array consisting of severalradiating elements. Each processing channel in transmission modecomprises an analogue radiofrequency chain RF₁, RF₂ which consists ofone or more filter(s), of one or more amplifier(s) and optionally of oneor more mixer(s) for carrying out a frequency transposition of thesignal. Each radiating element is associated with a transmissionchannel. The payload CU thus exhibits multichannel operation. In FIG. 3,a payload comprising two transmission channels has been represented, butthe number of channels is in general bigger.

FIG. 4 details, on a flowchart, the steps of carrying out a method forestimating and correcting the pointing error of the antenna ANT.

In a first step 401, a test signal, similar to that describedpreviously, is generated in the set of digital integrated circuits PN.For example, it can be stored digitally in a memory MEM and thendispatched to the digital-to-analogue converters DAC₁, DAC₂ of eachprocessing channel.

To be able to separate the transmission channels when the test signal isreceived on the ground, a multiple access technique is used to transmitthe test signal on the various transmission channels. The accesstechnique is, for example, a spreading code division multiple accesstechnique (of CDMA type), in this case the digital test signal generatedis spread with a different spreading code for each transmission channel.Another possible access technique is a frequency division multipleaccess technique (of FDMA type) which consists in modulating the testsignal on a different frequency for each transmission channel. Thedigital signals can be precomputed and stored in a memory or a registerMEM.

The test signal is thus dispatched on a set of the transmission channelsof the antenna ANT to at least two ground stations ST₁, ST₂ in twodifferent directions θ_(A), θ_(B).

In each of the ground stations ST₁, ST₂, the signals are separated 402by transmission channels as a function of the access technique (forexample of CDMA, FDMA or other type) used and are then digitized 403.

Thereafter, steps 203, 204, 205 described in FIG. 2 are appliedidentically to determine an estimate of the pointing error of theantenna ANT.

The computation of the estimate of the pointing error is carried outeither in one of the ground stations ST₁, ST₂, or in ground equipmentwhich may or may not be connected to the ground stations ST₁, ST₂.

The pointing error is thereafter dispatched 404 to the satellite via asecure link LS for correction of the pointing of the antenna.

1. A system for estimating a pointing error of an antenna (ANT) of asatellite, the satellite comprising a payload (CU) comprising amultichannel transmitter or receiver comprising a multichannel antenna(ANT), one analogue processing chain per channel and a set of digitalintegrated circuits (PN), the system comprising an estimation device(EST), implemented aboard the satellite or in a ground station, forestimating a pointing error of the antenna, the device for estimating apointing error being configured to: acquire, for at least two channelsof the transmitter or of the receiver, at least two test signals, eachtest signal having been transmitted or received by the antenna along adifferent direction (θ_(A), θ_(B)) from the viewpoint of the satellite,for at least one pair of acquired test signals, determine, for each testsignal, a relative complex gain between the test signal received ortransmitted respectively on two distinct channels, determine acomparative measurement between the two test signals from either theratio between the two relative complex gains and/or the differencebetween the phases of the two relative complex gains, determine apointing error (dθ) of the antenna on the basis of the comparativemeasurement, of the expected directions of transmission or of receptionof the test signals (θ_(A), θ_(B)) and of a model of the gain of theantenna for each channel and in a plurality of directions.
 2. The systemfor estimating a pointing error of an antenna according to claim 1,wherein each test signal is composed of at least one spectral line. 3.The system for estimating a pointing error of an antenna according toclaim 1, wherein the estimation device (EST) for estimating a pointingerror is configured to determine a plurality of estimates of a pointingerror on the basis of several pairs of test signals transmitted orreceived in different directions or several different pairs of distinctchannels or several frequencies of test signals.
 4. The system forestimating a pointing error of an antenna according to claim 1, whereinthe estimation device (EST) for estimating a pointing error isconfigured to determine a pointing error of the antenna by: determining,on the basis of the model of the gain of the antenna, a model ofcomparative measurement between the two test signals, dependent on apointing error of the antenna, on the frequency and on the expecteddirections of transmission or of reception of the test signals (θ_(A),θ_(B)), searching for the value of the pointing error which makes itpossible to minimize the difference between the comparative measurementand the comparative measurement model taken at this value.
 5. The systemfor estimating a pointing error of an antenna of a satellite accordingto claim 1, wherein the payload (CU) comprises a multichannel receiverand the estimation device (EST) for estimating a pointing error isconfigured to receive, for at least two reception channels, a digitizedtemporal sample sequence of at least two test signals, the temporalsample sequence being acquired simultaneously from the various channelsin the set of digital integrated circuits (PN).
 6. The system forestimating a pointing error of an antenna of a satellite according toclaim 5, furthermore comprising at least two test ground stations (ST₁,ST₂), each test ground station being configured to transmit a testsignal towards the satellite.
 7. The system for estimating a pointingerror of an antenna of a satellite according to claim 5, wherein eachtest ground station is configured to transmit the test signals in turnon one and the same frequency or simultaneously on distinct frequencieswhich are sufficiently close to be affected by the same errors.
 8. Thesystem for estimating a pointing error of an antenna of a satelliteaccording to claim 1, wherein the payload comprises a multichanneltransmitter, the estimation device (EST) for estimating a pointing errorbeing implemented in a ground station, each test signal received by adifferent ground station being transmitted by the antenna along adifferent direction of transmission (θ_(A), θ_(B)) from the viewpoint ofthe satellite.
 9. The system for estimating a pointing error of anantenna of a satellite according to claim 8, furthermore comprising atleast two test ground stations (ST₁, ST₂), each test ground stationbeing configured to: receive a test signal transmitted by the satellitealong a different direction from the viewpoint of the satellite,separate the test signal received on the ground into several signalscorresponding to the respective transmission channels of the satellite,dispatch, to the device for estimating a pointing error, the respectivetest signals received for at least two distinct transmission channels.10. The system for estimating a pointing error of an antenna of asatellite according to claim 8, wherein the system is configured toapply a procedure for multiple access to the test signals transmitted bythe satellite.
 11. A method for estimating a pointing error of anantenna of a satellite, the satellite comprising a payload comprising amultichannel transmitter or receiver comprising a multichannel antenna,one analogue processing chain per channel and a set of digitalintegrated circuits, the method comprising the steps of: acquiring, forat least two channels of the transmitter or of the receiver, at leasttwo test signals, each test signal having been transmitted or receivedby the antenna along a different direction (θ_(A), θ_(B)) from theviewpoint of the satellite, for at least one pair of acquired testsignals, determining, for each test signal, a relative complex gainbetween the test signal received or transmitted respectively on twodistinct channels, determining a comparative measurement between the twotest signals from either the ratio between the two relative complexgains and/or the difference between the phases of the two relativecomplex gains, determining a pointing error (dθ) of the antenna on thebasis of the comparative measurement, of the expected directions oftransmission or of reception of the test signals and of a model of thegain of the antenna for each channel and in a plurality of directions.12. The method for estimating a pointing error of an antenna of asatellite according to claim 11, furthermore comprising a step ofcorrecting the pointing of the antenna (ANT) on the basis of thepointing error (dθ).